A new Steam client update has brought support for a variety of "Nintendo Online classic controllers", meaning you will be able to use the NES, SNES, Nintendo 64, and Mega Drive controllers on the platform, despite these only being on sale for Switch Online members.
Considering the lack of inputs these devices feature, don't expect to be able to play every game on Steam with them, but at the same time, there should be an array of titles that can now be played with these iconic controllers.
On top of this, the update notes states that the Qanba Obsidian and Dragon Arcade Joysticks are supported as well, as is the HORI Fighting Stick mini 4. Various other DualSense controller updates have been introduced, which should tackle firmware updates and improved rumble emulation.
The client update did also feature a variety of other technology improvements, which you can read all about here.